A group of researchers released a paper today that describes a new class of serious vulnerabilities in PGP (including GPG), the most popular email encryption standard. The new paper includes a proof-of-concept exploit that can allow an attacker to use the victim’s own email client to decrypt previously acquired messages and return the decrypted content to the attacker without alerting the victim. The proof of concept is only one implementation of this new type of attack, and variants may follow in the coming days.

Because of the straightforward nature of the proof of concept, the severity of these security vulnerabilities, the range of email clients and plugins affected, and the high level of protection that PGP users need and expect, EFF is advising PGP users to pause in their use of the tool and seek other modes of secure end-to-end communication for now.

Because we are awaiting the response from the security community of the flaws highlighted in the paper, we recommend that for now you uninstall or disable your PGP email plug-in. These steps are intended as a temporary, conservative stopgap until the immediate risk of the exploit has passed and been mitigated against by the wider community. There may be simpler mitigations available soon, as vendors and commentators develop narrower solutions, but this is the safest stance to take for now. Because sending PGP-encrypted emails to an unpatched client will create adverse ecosystem incentives to open incoming emails, any of which could be maliciously crafted to expose ciphertext to attackers.

While you may not be directly affected, the other participants in your encrypted conversations are likely to be. For this attack, it isn’t important whether the sender or the receiver of the original secret message is targeted. This is because a PGP message is encrypted to both of their keys.

At EFF, we have relied on PGP extensively both internally and to secure much of our external-facing email communications. Because of the severity of the vulnerabilities disclosed today, we are temporarily dialing down our use of PGP for both internal and external email.

Our recommendations may change as new information becomes available, and we will update this post when that happens.

How The Vulnerabilities Work

PGP, which stands for “Pretty Good Privacy,” was first released nearly 27 years ago by Phil Zimmermann. Extraordinarily innovative for the time, PGP transformed the level of privacy protection available for digital communications, and has provided tech-savvy users with the ability to encrypt files and send secure email to people they’ve never met. Its strong security has protected the messages of journalists, whistleblowers, dissidents, and human rights defenders for decades. While PGP is now a privately-owned tool, an open source implementation called GNU Privacy Guard (GPG) has been widely adopted by the security community in a number of contexts, and is described in the OpenPGP Internet standards document.

The paper describes a series of vulnerabilities that all have in common their ability to expose email contents to an attacker when the target opens a maliciously crafted email sent to them by the attacker. In these attacks, the attacker has obtained a copy of an encrypted message, but was unable to decrypt it.

The first attack is a “direct exfiltration” attack that is caused by the details of how mail clients choose to display HTML to the user. The attacker crafts a message that includes the old encrypted message. The new message is constructed in such a way that the mail software displays the entire decrypted message—including the captured ciphertext—as unencrypted text. Then the email client’s HTML parser immediately sends or “exfiltrates” the decrypted message to a server that the attacker controls.

The second attack abuses the underspecification of certain details in the OpenPGP standard to exfiltrate email contents to the attacker by modifying a previously captured ciphertext. Here are some technical details of the vulnerability, in plain-as-possible language:

When you encrypt a message to someone else, it scrambles the information into “ciphertext” such that only the recipient can transform it back into readable “plaintext.” But with some encryption algorithms, an attacker can modify the ciphertext, and the rest of the message will still decrypt back into the correct plaintext. This property is called malleability. This means that they can change the message that you read, even if they can’t read it themselves.

To address the problem of malleability, modern encryption algorithms add mechanisms to ensure integrity, or the property that assures the recipient that the message hasn’t been tampered with. But the OpenPGP standard says that it’s ok to send a message that doesn’t come with an integrity check. And worse, even if the message does come with an integrity check, there are known ways to strip off that check. Plus, the standard doesn’t say what to do when the check fails, so some email clients just tell you that the check failed, but show you the message anyway.

The second vulnerability takes advantage of the combination of OpenPGP’s lack of mandatory integrity verification combined with the HTML parsers built into mail software. Without integrity verification in the client, the attacker can modify captured ciphertexts in such a way that as soon as the mail software displays the modified message in decrypted form, the email client’s HTML parser immediately sends or “exfiltrates” the decrypted message to a server that the attacker controls. For proper security, the software should never display the plaintext form of a ciphertext if the integrity check does not check out. Since the OpenPGP standard did not specify what to do if the integrity check does not check out, some software incorrectly displays the message anyway, enabling this attack.

This means that not only can attackers get access to the contents of your encrypted messages the second you open an email, but they can also use these techniques to get access to the contents of any encrypted message that you have ever sent, as long as they have a copy of the ciphertext.

What's Being Done to Fix this Vulnerability

It’s possible to fix the specific exploits that allow messages to be exfiltrated: namely, do better than the standard says by not rendering messages if their integrity checks don’t check out. Updating the protocol and patching vulnerable software applications would address this specific issue.

Fixing this entirely is going to take time. Some software patches have already begun rolling out, but it will be some time before every user of every affected software is up-to-date, and even longer before the standards are updated. Right now, information security researchers and the coders of OpenPGP-based systems are poring over the research paper to determine the scope of the flaw.

We are in an uncertain state, where it is hard to promise the level of protection users can expect of PGP without giving a fast-changing and increasingly complex set of instructions and warnings. PGP usage was always complicated and error-prone; with this new vulnerability, it is currently almost impossible to give simple, reliable instructions on how to use it with modern email clients.

It is also hard to tell people to move off using PGP in email permanently. There is no other email encryption tool that has the adoption levels, multiple implementations, and open standards support that would allow us to recommend it as a complete replacement for PGP. (S/MIME, the leading alternative, suffers from the same problems and is more vulnerable to the attacks described in the paper.) There are, however, other end-to-end secure messaging tools that provide similar levels of security: for instance, Signal. If you need to communicate securely during this period of uncertainty, we recommend you consider these alternatives.

We Need To Be Better Than Pretty Good

The flaw that the researchers exploited in PGP was known for many years as a theoretical weakness in the standard—one of many initially minor problems with PGP that have grown in significance over its long life.

You can expect a heated debate over the future of PGP, strong encryption, and even the long-term viability of email. Many will use today’s revelations as an opportunity to highlight PGP’s numerous issues with usability and complexity, and demand better. They’re not wrong: our digital world needs a well-supported, independent, rock-solid public key encryption tool now more than ever. Meanwhile, the same targeted populations who really need strong privacy protection will be waiting for the steps they can take to use email securely once again.

We’re taking this latest announcement as a wake-up call to everyone in the infosec and digital rights communities: not to pile on recriminations or criticisms of PGP and its dedicated, tireless, and largely unfunded developers and supporters, but to unite and work together to re-forge what it means to be the best privacy tool for the 21st century. While EFF is dialing down our use of PGP for the time being (and recommend you do so too) we’re going to double-down on supporting independent, strong encryption—whether that comes from a renewed PGP, or from integrating and adapting the new generation of strong encryption tools for general purpose use. We’re also going to keep up our work improving the general security of the email ecosystem with initiatives like STARTTLS Everywhere.

PGP in its current form has served us well, but “pretty good privacy” is no longer enough. We all need to work on really good privacy, right now.

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